The maximum data packet allowed to pass an interface of a network device is dependent upon hardware configuration, transmission protocol and other factors of the interface. If the length of a data packet exceeds the maximum data packet length allowed to pass an interface of a network device, the data packet will be fragmented into several fragments, which are encapsulated respectively into several fragment packets with lengths no more than the maximum data packet length and transmitted to a destination node for recombination; and this process can be referred to as fragmentation, and the packet obtained from the fragmentation can be referred to as a fragment packet.
The maximum link layer payload allowed to pass a network device without any fragmentation can be referred to as a Maximum Transmission Unit (MTU). The maximum link layer payload which can be transmitted from a source host to a destination host without any fragmentation can be referred to as a Path Maximum Transmission Unit (PMTU). And in general, the PMTU of a data packet transmission path is equal to the minimum MTU among all of device interfaces on the path.